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I have a 70s Miyata mostly equipped with original Dura-Ace parts. The shifter is a Shimano Crane, which is pretty much a modern design. The cables are modern SIS-type. Right now the wheel I have on it is using an 8-speed cassette. Is there any reason I couldn't buy an 8-speed brifter or barend and have indexed shifting?

Is your cassette Shimano? If so you could use any Shimano 8-speed shifters. But if you use Dura-Ace 8-speed shifters you'll need a Dura-Ace rear derailleur.

If your cassette is Shimano you could replace the cassette with a 9 or 10-speed cassette and buy the corresponding shifters and chain. Nine speed components may be more compatible with your old crankset and derailleurs than 10-speed.

It's really easy to spend too much money on upgrades. I'm just saying that there are other possibilities.

I suppose it's possible it might work, but it would more serendipitous than not. Indexed shifting systems are designed as a package: shifters, derailleurs, cluster. A specific amount of cable pull from the shifter moves the derailleur precisely the right distance to move the chain from one cog to the next. If the cog spacing is off, or the derailleur geometry different enough to cause a difference in how far the cable pulls the derailleur, it won't work, or at a minimum it won't work well.

It sounds like your shifters and cluster are relatively modern, but the "Crane" derailleur dates from the mid -1970s. There's no guarantee its geometry will work with indexing, and it lacks the floating upper pulley that allows the chain to center itself on the cog. But you have it already (apparently) so it can't hurt to try. I suspect you'll end up replacing it with a modern derailleur to get the indexing to work satisfactorily.

I guess John got at my real question: what fundamental changes have there been in derailleurs that this might not work.

The derailleur was one of the earliest ones with a pretty modern geometry, so it's probably worth a shot. That pulley might be an issue, though. Probably swappable, but modern derailleurs are pretty easy to fish up, as well.

This will not shift well because this derailleur was made before SunTour's patent on the slant parallelogram expired. Index shifting doesn't work well in derailleurs than cannot maintain an accurate chain gap, which is why Campy's early attempts at index shifting were less than stellar...

I think you'd be better off selling the Crane on ebay and buying a Sora rear derailleur, instead.